Abstract

Sunnamite, taaffeite, beryllian sapphirine, niobian rutile, chrysoberyl, and wagnerite occur in sillimanite-rich, medium-grained segregations in two pegmatites of probable late Archean age which cut granulite-facies rocks near McIntyre Island (67°22′S, 49°05′E) in Casey Bay, Antarctica. Antarctic surinamite contains 30.9–32.1 wt.% SiO2, 33.7–37.3 wt.% Al2O3, 9.1–13.4 wt.% FeO, 0.01–0.2 wt.% MnO, and 16.4–18.4 wt.% MgO, 0.5–1 wt.% Be, and 0.66±0.2 wt.% H2O. Assuming water is absent, a constant BeO content of 3.5 wt.%, and possible Fe3+ for Al substitution, surinamite analyses recast to 32 oxygens contain 22 cations. Antarctic taaffeite contains 68.5–69.3 wt.% Al2O3, 9.7 wt.% FeO, 10.6–11.0 wt.% MgO, 4.7–5.2 wt.% ZnO, and 0.5–1 wt.% Be. Cell parameters are a = 5.6804 (2)Å, c = 41.104 (2)Å, suggesting that this taaffeite is the 9R polytype. Sapphirine contains 20.3 wt.% SiO2, 51.1 wt.% Al2O3 and 0.5–1 wt.% Be, indicating substitution of Be for Al: Be + Si = 2Al. Associated garnet contains 33 to 42 mole % pyrope and associated cordierite, 1 wt.% Na2O. Stable mineral assemblages in the segregations at the time the pegmatites crystallized are quartz–sillimanite–surinamite–biotite–orthopyroxene–garnet, quartz–surinamite–taaffeite, sillimanite–garnet–biotite–surinamite–taaffeite–sapphirine, and sillimanite–garnet–biotite–chrysoberyl–surinamite. Temperatures and pressures at the time of pegmatite emplacement are estimated to have been 800–900°C and 7–8 kbar. The Archean beryllian pegmatites are interpreted to be of magmatic origin and may be related to chamockitic plutonic rocks such as the body exposed on Tange Promontory 70 km to the west.

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